This invention relates to a process for producing a polymer or copolymer of acrylic acid or a water-soluble derivative thereof. More in detail, the present invention relates to a process for producing a polymer or copolymer of acrylic acid or a water-soluble derivative thereof using a special polymerization initiator in a short time with high efficiency.
Water-soluble vinyl polymers and copolymers such as poly(acrylic acid) or salts thereof, polyacrylamide, polyvinylpyrrolidone, and the like have been noticed for their utilities. For example, polyacrylates and copolyacrylates are widely used, for example, as a thickener, a flocculant, a highly water-absorbing resin which is used in large amounts for disposal diaper, etc. Acrylamide polymers are widely used, for example, as a flocculant, a thickener, a yield improving agent for paper-making, a fluidity adjusting agent for oil salvage, etc. Such water-soluble polymers and copolymers used for such purposes have been obtained by polymerizing or copolymerizing acrylic acid, acrylamide, or derivatives thereof as a monomer or monomers in an aqueous solution or a suspension of water and water-insoluble organic solvent using a redox polymerization initiator which is a combination of an oxidizing agent (e.g. a peroxide) and a reducing agent, a peroxide polymerization initiator such as benzoyl peroxide, lauroyl peroxide, potassium persulfate, etc. an azo polymerization initiator such as 2,2'-azobis(2-amidinopropane) dihydrochloride,etc.
But these known polymerization processes have various problems therein. For example, in the case of using the redox polymerization initiator and the peroxide polymerization initiator, since peroxides per se used therein are generally unstable for impact, heating, etc., there is a danger of fire, explosion, and the like during the storage of the peroxides or at the time of polymerization. Further, many problems also arise when applied to industrial production, for example, a long time being necessary for polymerization due to a long induction period, the temperature control at the time of reaction being difficult due to easy self-induced decomposition, and the like. In addition, according to the polymerization process using such polymerization initiators, since the polymer obtained has a small molecular weight, there is a problem of failing to obtain a sufficient aggregating effect when used as a flocculant.
On the other hand, in a process using an azo polymerization initiator such as 2,2'-azobis(2-amidinopropane) dihydrochloride, there is a merit of easy temperature control due to precise primary decomposition, not showing self-induced decomposition unlike the peroxides, but-there is also a problem of easily bringing about the so-called dead-end phenomenon wherein the polymerization is stopped due to consumption of the polymerization initiator during the polymerization under preferable polymerization conditions.
As azo polymerization initiators, the use of 2,2'-azobis(2-methylpropionamidoxime) is disclosed, for example, in Japanese Patent Unexamined Publication (JP-A) Nos. 62-172007, and 61-223009 (E.P 196588), U.S. Pat. Nos. 4,528,347, and 4,644,042 (=JP-A 61-60707) for polymerization of monoallylamine (CH.sub.2 .dbd.CHCH.sub.2 NH.sub.2) which is a basic allyl compound and in U.S.S.R. SU 927,802 for polymerization of acrylonitrile, which is not water-soluble. No prior art reference discloses the use of such an azo initiator for polymerization of acidic or neutral water-soluble vinyl compounds such as acrylic acid, acrylamide, etc.